Secondary electron emitter



ov, 2, 1950 L. E. cHEEsMAN ETAL, 2,530,946

SECONDARY ELECTRON EMITTER 2 Sheets-Sheet l Filed April 2, 1949 PLATINUMTARGE T VOLTAGE THGET. VOL TAGE' L i m55/v INI/EUR5 A TTOR/VEV Nom 2l,E950 L. E. CHEESMAN ET AL SECONDARY ELEcTRoN EMITTER 2 Sheets-Sheet 2Filed April 2, 1949 TARGET TREATM-NT M ,4T noo/w TEMPERATURE N- AFTER700%'. F0@ so MINUTES (HOT) P- AFTER 0500. Fon /5 Mmc/Tes ,4A/0

THE/v AT 700c, Fon a0 MINUTES (H0 T) s- AFTEH 050 c. Fok 15 MINUTES AlvaTHEN AT 000c. Fon a0 MINUTES (com) 00 BOO |200 |600 2000 400 2800 TARGA?T VOL TAGE ATTORNEY Patented Nov. 21, 1950 SECONDARY ELECTRON EMITTERLeonard E. Cheesrnan, Clifton, and Hallam Mendenhall, Summit, N. J.,assignors to Bell Telephone Laboratories, incorporated, New York, N. Y.,a corporation of New York Application April 2, 1949, Serial No. 85,161

18 Claims. (Cl. Z50-174) This invention relates to secondary electronemissve electrodes and to methods of making such electrodes.

One general object of this invention is to improve electron dischargedevices including one or more secondary electron emissive targetelectrodes. lvlore specically, objects of this invention are to obtainlarge secondary emission ratios for such electrodes, increase thestability and operating life of secondary electron emissive elements andenable attainment oi prescribed emissive ratios for such elements.

It has been discovered that oxides of semiconductive materials, such assilicon and germanium, which oxides normally are 4highly insulating butvery poorly secondary electron emissive, can be treated so that highemission ratios, for example of the order of five, are obtainabletherefor. More specifically, it has been discovered that such oxides inthe form of layers or thin lms upon a base oi metal containing smallamounts or traces of impurities can be rendered highly secondaryelectron emissive by proper heat treatment oi the oxide-base unit.Further, secondary emission ratios within a range of values can berealized by correlation of the thickness of the oxide lm and the timeand temperature of the heat treatment.

In one illustrative embodiment of this invention, a target electrodecomprises a film of silicon dioxide upon a base oi commercially pureplatinum. For a nlm thickness of 1GO angstr-om units and heat treatmentof the oxide-base unit at 760 C, for one-half hour, a secondary emissionratio of substantially 4.6 at a primary electron voltage of about 450volts is obtained.

In ail-other illustrative embodiment of this invention, a targetelectrode comprises a layer of germanium oxide upon a commercially pureplatinum base. A layer of one milligram per square centimetercorresponding to a thickness of about lll-4 centimeters, heat treated atabout 700 C. for about one hour, exhibits a secondary electron emissionratio of substantially 6 at a primary electron voltage of about 650volts.

Comparable emission ratios are obtained for silica and germania lms uponchemically pure platinum. v

' The invention and the various features thereof will be understood moreclearly and fully from the following detailed description with referenceto the accompanying drawing in which:

Fig. l is a diagram of an electron discharge device illustrative ofthose in which target electrodes constructed in accordance with thisinvention may be utilized;

Fig. 2 is a graph illustrating the relation between the secondaryelectron emission ratio of a 100angstrom silicon dioxide iilm on acommercially pure platinum base, and the primary voltage, for variousheat treatments of the film-base unit;

Fig. 3 is another graph illustrating the secondary emissionratio-primary voltage relation for silicon dioxide films ci severalthicknesses on commercially pure and chemically pure platinum fordirerent heat treatments;

4 is a perspective view of apparatus for producing silicon dioxide filmsupon a metal base;

Fig. 5 is a perspective view of the crucible in eluded in the apparatusshown in Fig. 4; and

Fig. 6 is a graph showing secondary emission characteristics forgermanium oxide upon a platinum base.

Referring now to the drawing, the electron discharge device illustratedin Fig. l comprises a highly evacuated enclosing vessel it having thereatarget electrode l l and an electron gun which,

iol' purposes of simplicity, is shown as comprising an indirectly heatedcathode l2 and an accelerating electrode i3 maintained positive withrespect to the cathode by a source EQ. It will be understood that thegun may be of Well-known construction and projects an electron streamtoward the target electrode ii. The target electrode l i is maintainedat a positive potential relative to the electron gun by a direct-currentsource i5, poled as shown, the potential being variable if desired tocontrol the energy of the primary electrons striking the targetelectrode. Opposite the target electrode is a collector electrode i6which may be cylindrical, as shown, and is biased positive relative tothe target electrode by a direcit-current source il. A load I3 isconnected between the electrodes il and l, the current to the load beingdetermined by the number of secondary electrons emanating from thetarget and received by the collector electrode l.

The target electrode l i comprises, as indicated in Fig. 1, a metallicbase, described hereinafter, having on the face thereof toward theelectron gun i2, i3, a film of substantially pure silicon dioxide orgermanium oxide. Such a lm of silicon dioxide may be formed by vapordeposition of the iilm material, under Vacuum, upon the base. Suitableapparatus for such deposition is illustrated in Fig. 4 and comprises avessel 20 evacuated during the process to or the order of lil-5millimeters of mercury. Mounted within the vessel 2o and upon a base 2 iare a pair of brackets 22 upon which the base member of the target isremovably held, as by spring clips 23. In juxtaposition to the targetbase is a basket or Crucible 2li of pure refractory material, forexample of tungsten wire and conical as shown, which is Supported byleading-in conductors 25 extending from terminals 26 affixed to the baseEl. rThe Crucible or basketBi has therein a predetermined quantity ofcrushed silicon dioxide, indicated at 3 2l' in Fig. 5, the quantitydetermining the thickness of the lm deposited.

In the process, the crucible is heated, by passage of currenttherethrough, to a temperature sufficient to vaporize the materialtherein. For example, in the case of silica, the temperature may beabout 2000 C. which is sufficiently low to preclude any substantialevaporation of the tungsten, whereby substantially pure silicon dioxideis deposited upon the target base. The time required for evaporation anddeposition of a silica nlm of the order of 50 to 200 angstroms thick isabout seconds. As noted heretofore, the thickness of the deposited filmis determined by quantity of charge. In one illustrative case, a chargeof 2 milligrams of crushed silica produces a layer of a uniformthickness of 100 angstromsover an area of 6 square centimeters upon thetarget base, the silica being evaporated by heating of the tungsten wirecrucible at 2000 C. for 15 seconds, the crucible having an includedangle of about degrees.

After deposition of the film upon the base, the target element is heatedin an inert atmosphere or vacuum, for example by passage of a currenttherethrough, or in a furnace. The effects of such treatment of targetscomprising lms of silica upon a platinum base will be appreciated from aconsideration of Figs. 2 and 3. In the former, the relationships betweensecondary emission ratio and primary electron voltage for atargetcomprising a 100angstrom thick nlm on a base of commercially pureplatinum after different heat treatments, are indicated. Themeasurements represented graphically were made with the target at roomtemperature.

It willbe noted from Fig. 2 that the primary electron Voltage at whichthe maximum secondary emission ratio obtains is substantially the same,about 450 Volts, for all the cases represented but that the magnitude ofthe emission ratio varies markedly with the previous target treatment.Specically, it will be noted that for the target at room temperature, i.e. without heat treament (curve A), the maximum emission ratio isslightly below 3, whereas for a target previously heated one-half hourat 760 C. (curve C), the maximum emission ratio is about 4.5, anincrease of over 50 per cent. The treatment at a temperature of about'760 C. results in the optimum ratio for, as indicated by the curves B'and. D to G, treatment at lower or higher temperatures results in asmaller emission ratio. ylreatment at the higher temperatures of atarget previously heated to about 760 C. results in deactivation ofthetarget.

It appears that the enhanced secondary emission ratio obtained by theheat treatment as described hereinbove is attributable to activation ofthe silica by donor impurities present in the platinum base. Analysesshow that commercial platinum contains a number of impurities in thefollowing approximate ranges:

Tin, palladium-0.01-0.3 per cent Copper-trace, less than 0.3 per centGold, iron, manganese, nickel, leadslight trace,

less than 0.005 per cent Barium, manganese, siliconvery slight trace,

less than .001 per cent and that chemically pure platinum, in general,contains the same impurities but in amounts an order of magnitude lessthan in. commercially pure. platinum. The ranges areindicated below:

Tin, palladium-trace, less than 0.03 per cent Copper-slight trace, lessthan 0.005 per cent Gold, barium, manganese, silicon-very slight trace,less than .001 per cent This explanation of the enhanced emission ratiosis consistent with results obtained, some of which are illustrated inFig. 3. In this figure, curve H shows the emission ratio-primaryelectron voltage relation for a target comprising a -angstrom filmofsilica upon a base of commercially pure platinum, without heat treatmentand curve H shows the relation for the same target after heating at 750C. for one-half hour. These, it will be noted, correspond to curves Aand C of Fig. 2. Curve J indicates the relation mentioned for a targetof a 50-angstrom film upon a chemically pure platinum base, without heattreatment; curve J is for this same target after heat treatment at 750C. for 20 hours. It will be noted that the treatment results in amaximum emission ratio substantially the same as that for the targetrepresented by curve H.

Curves K and K are for a target comprising aV ZOO-angstrom film ofsilicon dioxide upon a base of commercially pure platinum, the basehaving been used previously in a target having a silica film thereonheated to 750 C. and then deactivated. Curve K indicates the emissioncharacteristic for the ZOO-angstrom target Without heat treatment; curveK' shows this characteristic for the same target after heating it for 14hours at about 750 C.

As indicated hereinabove, chemically pure platinurn contains lesseramounts of impurities than commercially pure material. It is to beexpected, therefore, that the former has available fewer donors whichcan diffuse into the silica to activate it so that longer heat treatmentof the target is necessary to produce emission ratios comparable to thatfor a base of commercially pure platinum. This is supported by curves Jand J.' of Fig. 3.

Similarly, in the case of a seconduse of a base, it is to be expectedthat the amounts of impurities remaining for diifusion intoy the silicaare less than in the first use so that longer heat treatment is requiredto enhance the secondary emissive ratio. This issubstantiated by curvesK and K.

As has been indicated hereinbefore, germanium oxide also may beutilized, in accordance with this invention. The lm or layer may beformed upon a commercially or chemically pure platinum base byvaporization of germanium dioxide in the same manner as described abovefor silicon dioxide. It may be formed also by spraying germanium dioxidein a suitable binder, for example nitro-cellulose, upon the base andremoving the binder by heating the-base-iilm unit.

Fig. 6 illustrates the secondary emissioneprimary voltagecharacteristics of a target having thereona lm or layer of l milligramper square centimeter of germaniumdioxide, appliedf to a commerciallypure platinum base by spraying as noted hereinabove. Curve M illustratesthe secondary emission of the target at room temperature, i. e. beforeheat treatment, the curve N after heating at '700 C. for 30 minutes,before activation is complete. In both cases, it will be'noted, thesecondary emission ratios are small.

Curve P shows the characteristics-of the target, measured at 700 C.,after heating at`850 C. for. lminutes and-then atYOQ," C. for30;minutes.

The maximum secondary emission ratio of about 6.4 at a primary electronvoltage of about 600 volts is to be noted particularly. The secondaryemission characteristics for the target at room temperature afterheating at 850 C. for minutes and then at 600 C. for 30 minutes areshown by curve S.

It is evident from Fig. 6 that heating of the germanium oxide-platinumtarget enhances the secondary emission and that very marked increase inthis ratio results from heat treatment at the order of 600 C. to 850 C.These increases may be explained, as in the case of silicon dioxide, byactivation of the germanium oxide by migration thereinto of impuritydonors from the platinum.

Although speciiic embodiments of the invention have been shown anddescribed, it will be understood that they are but illustrative and thatvarious modifications may be made therein without departing from thescope and spirit of this invention.

What is claimed is:

1. The method of making a secondary electron emittei` which comprisesapplying to a base of substantially pure metal having only a smallfraction of one per cent of impurity therein, a film of an oxideselected from the group consisting of silicon dioxide and germaniumoxide, and heating the composite unit to effect diffusion of theimpurity from the base into the oxide.

2. rFhe method of enhancing the secondary electron emissive ratio of anoxide selected from the group consisting of silicon dioxide andgermanium oxide, which comprises heating the r,

oxide in the presence of an impurity to diffuse the impurity thereinto.

3. The method of making a secondary electron emitter which comprisesapplying to a base of platinum having a fraction of one per cent ofimpurity therein, 'a coating of an oxide selected from the groupconsisting of silicon dioxide and germanium dioxide, and heating thecomposite unit at a temperature of the order of 700 C.

4. The method of making a secondary electron emitter which comprisesapplying to a platinum base a lm of silicon dioxide between 50 and 200angstroms thick, and heating the composite body at a temperature ofsubstantially 760 C. for between about one-half and about hours.

5. The method of making a secondary electron emitter which comprisesforming a lm of silicon dioxide on a base of commercially pure platinum,and heating the composite unit to a temperature of the order of 760 C.

6, The method of making a secondary electron emitter which comprisesapplying a lm of silicon dioxide about 100 angstroms thick to a base ofcommercially pure platinum, and heating the composite unit forapproximately onehalf hour at a temperature of substantially '760 C.

7. The method of making a secondary electron emitter which comprisesapplying a film of silicon dioxide to a Vbase of chemically pureplatinum, and heating 'the composite unit at about 760 C.

8. A secondary electron emissive target for electron discharge devicescomprising a substantially pure metal base having therein a fraction oI"one per cent of impurity, and a film on said base of an oxide selectedfrom the group consisting of silicon dioxide and germanium dioxide.

9. A secondary electron emissive target for electron discharge devicescomprising a platinum base, and a coating of silicon dioxide thereon.

10. A secondary electron emissive target for electron discharge devicescomprising a platinum base, and a film of silicon dioxide between about50 and 100 angstroms thick on said base.

11. A secondari7 electron emissive target for electron discharge devicescomprising a base of commercially pure platinum, and a nlm of silicondioxide between about 50 and 100 angstroms thick upon said base.

12. A secondary electron emissive target for electron discharge devicescomprising a base of chemically pure platinum, and a nlm of silicondioxide of the order of 50 angstroms thick upon said base.

13. The method of making a secondary electron emitter which comprisesforming a layer of germanium oxide upon a base of metal having animpurity therein, and heating the oxide-base element at a temperature ofthe order of 600 C. to 850 C.

14. The method of making a secondary electron emitter which comprisesforming a layer of germanium oxide upon a base of platinum containingimpurity, and heating the composite unit at a temperature of about 850C, for about 15 minutes.

15. The method of making a secondary electron emitter which comprisesforming a film of germanium dioxide upon a base of commercially pureplatinum, heating the composite unit at a temperature of the order of850 C. for about l5 minutes, and then heating said unit at a temperatureof the order of 600 C. to '700 C. for about 30 minutes.

16. A secondary electron emissive electrode for electron dischargedevices comprising a metal base, and a layer of germanium oxide on saidbase.

17. A secondary electron emissive electrode for electron dischargedevices comprising a base of platinum, and a layer of germanium oxide onsaid base.

18. A secondary electron emissive electrode for electron dischargedevices comprising a base of commercially pure platinum, and a layer ofgermanium dioxide on said base.

LEONARD E. CHEESMAN. HALLAM E. MENDENHALL.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,171,224 Rose Aug. 29, 1939FOREIGN PATENTS Number Country Date 214,262 Great Britain Aug. 21, 192451,148 Netherlands Oct. 15, 1941

8. A SECONDARY ELECTRON EMISSIVE TARGET FOR ELECTRON DISCHARGE DEVICESCOMPRISING A SUBSTANTIALLY PURE METAL BASE HAVING THEREIN A FRACTION OFONE PER CENT OF IMPURITY, AND A FILM ON SAID BASE OF AN OXIDE SELECTEDFROM THE GRUP CONSISTING OF SILICON DIOXIDE AND GERMANIUM DIOXIDE.